The touch or contactless interfaces of portable systems such as telephones, smartphones, tablets, computers, etc. are frequently based on capacitive measurement techniques.
According to these techniques, the screen or the device is provided with electrodes, which are optionally transparent. When an object such as a finger comes close to these electrodes, an electrical coupling of capacitive type is created between this object which represents an electrical ground, and the electrodes. Measurement of this capacitive coupling thus allows the object to be located.
The progress towards contactless interfaces for which objects must be detected and located at relatively long distances from the surface of the interface requires the development of capacitive detection systems with very high sensitivity.
The capacitive “floating bridge” measurement technique, developed initially for very high accuracy measurement applications, proves to be particularly well adapted to this type of application.
This technique is for example described in the document FR 2 756 048 by Rozière. It makes it possible to eliminate all the parasitic capacitances which appear in more conventional capacitive measurement methods between the electrodes and their environment. To the extent that these parasitic capacitances, by definition unknown and fluctuating, are superimposed on the capacitance to be measured between an object and the electrodes, they significantly limit the sensitivity and the accuracy of measurement by conventional methods.
To this end, a part of the electronics, comprising the detection electrodes and the first stages of the measurement electronics which are the most sensitive, are made to float. Their reference electrical potential, or guard potential, is made to oscillate with respect to the general ground of the system for example using an oscillator which connects the ground and the guard. Thus, no part of the electronics of the sensitive stages is at the ground and it cannot create parasitic capacitances with the ground.
In order to be able to implement a capacitive measurement technique in a portable device such as a telephone or a tablet, it is imperative to be able to produce the associated electronics in the form of an integrated circuit of small overall dimensions and low power consumption.
It is known to produce measurement systems such as those disclosed in FR 2 756 048 in the form of printed or hybrid circuits with discrete components. On the other hand, the embodiments described in FR 2 756 048 cannot be produced in the form of a single integrated circuit.
One problem originates from the fact that the electronics comprise two distinct parts, with distinct reference potentials and oscillating with respect to one another. This generates the following constraints:                the two parts should not cause mutual interference, and in particular the grounded elements should not interfere with the floating elements;        the transfer of signals between the floating and grounded parts require decoupling components such as choke coils or optical couplers which cannot be integrated;        the electrical supplies of the floating part must be generated or transferred from grounded supply sources, which again requires decoupling components (choke coils, DC/DC converters) which are difficult to integrate.        
The document FR 2 893 711 by Rozière is known, which discloses a device for capacitive measurement by floating bridge based on an integrated circuit. However, in this case it is the whole of the integrated circuit that is supplied in floating mode, or in other terms at the guard potential. It is therefore necessary to add external components in order to produce the interfacing with the electronics referenced to ground, which is therefore also external to the measurement integrated circuit. Moreover, at least a part of the printed circuit on which it is fixed must also be referenced to the guard potential.
More generally, when capacitive electronics are implemented with a guard serving to minimize or eliminate parasitic capacitances, it is necessary to separate the guard area from the remainder of the electronics which is capable of generating stray capacitances. In all cases this constraint poses integration difficulties.
The purpose of the present invention is to propose an electronic device for capacitive measurement including a guard, which can be produced in the form of a single integrated circuit referenced to ground, so as to be able to be easily inserted onto a conventional electronic circuit.